The thermal neutral zone is the temperature range in which the calf does not need to expend extra energy to either keep warm or to stay cool. The range varies due to age, feed intake, body fat levels, and hair coat thickness.

We explored this issue in the article “Keep calves gaining during cold weather” found in the November 2008 issue on page 719.

Typically, the thermal neutral zone ranges from about 60° to 75° F. The lower end of this range is particularly critical for calves under 2 to 3 weeks of age because they have limited calf starter intake. With limited starter intakes, calves get little heat from rumen fermentation to help them keep warm.

Simply feeding more may seem like the most straightforward approach. However, that means that protein will be overfed since its requirement does not go up much during colder weather. A higher fat level milk replacer could be fed, but that means carrying another product and possibly overfeeding energy, depending on the weather, age, and calf starter intake.

The last option of using a fat supplement may provide the greatest flexibility but still requires some management decisions and practices. That option was explored using a 7 percent protein and 60 percent fat (MEn) in dry form. (Disclosure: I now work for a company that makes such a product).

Briefly, a 20 percent protein with 20 percent fat milk replacer was mixed at 10 ounces per 2 quarts. It was fed twice daily for a baseline in depiction of cold stress. That equates to 1.25 pounds dry milk replacer fed daily. The fat supplement was fed at 2 ounces at each milk replacer feeding which resulted in 0.25 pounds fed daily. This raised the solids level in milk replacer from 12.5 to 15 percent, still a reasonable solids feeding level.

Figure 1 shows the impact of 10° F shifts in average daily temperature on energy-based daily gains (EDG) for a 100-pound calf. Note that each 10° F drop in average daily temperature reduced daily gain by about 0.2 pound. That is a very significant reduction in calf performance for just a 10° F change!

Meanwhile, Figure 2 shows that an additional 20 pounds body weight would reduce daily gain by about 0.3 pound at 30° F or at any other 10° F interval. The fat supplement provided for 0.5 to 0.6 pound daily gain when added for any 10° F drop in temperature. But, the milk replacer’s contribution was lower simply due to the greater maintenance needs of another 20 pounds of body weight.

The combined energy contribution from both milk replacer and added fat barely maintained any daily gain at 0° F as shown in Figure 1. For a 120-pound calf, shown in Figure 2, milk replacer alone would result in negative daily gain. Meanwhile, the 140-pound calf netted out zero daily gain (absence of the blue bar) even when supplementing 0.25 pound of the fat.

Should be eating starter
With both 120- and 140-pound calves, there should be significant calf starter intake. The impact is considerable. For instance, a quarter pound of calf starter intake for the 120-pound calf would boost EDG by about 0.3 pound. By doubling intake to one-half pound, EDG goes to 0.5 pound at 20° F. Naturally, this EDG would be somewhat less, maybe 0.1 pound, with a 140-pound calf due to its greater body weight and maintenance energy requirements.

Don’t overlook water. Calves need about four times as much water intake as calf starter intake. And, in winter, this means feeding warm water to calves . . . up to three times daily.

How critical is warm water? A South Dakota State study showed that calves reduced rumen content temperature by an average of:
• 20° F when they drank 46° water
• 5° when they drank 63° water
• 2° to 3° when they drank water 80° to 99° water

In a recent summary of calf data over a 10-year period from the Cornell research herd, researchers found that a difference of 1 pound daily gain prior to weaning resulted in about 850 pounds more milk in the calf’s subsequent first lactation. Over three lactations, it totaled 2,280 pounds. When trying to understand why there was a wide range of 0.29 to 2.7 pounds daily gain before weaning, they found it was mostly related to colder weather. Calves born during winter months (average 32º F) consumed about 1.43 Mcal/d less energy above maintenance than calves born during warmer months (average 67º F). For each Mcal additional energy consumed above maintenance requirements, calves in colder weather produced 517 pounds more milk in first lactation. Over three lactations, an additional 2,000 pounds was produced due to higher energy intakes as calves. These are astonishing results being left on the table by not making adjustments for colder weather.

Southern climates not home free
In our southern climates, the rejoinder or understanding is that they do not have very cold weather. But if winter daily temperature averages just 50° F with a daily range of 40° to 60° F, calves just lost 0.2 pound daily gain. That could result in 170 pounds less milk in the first lactation and 400 pounds over the first three lactations.

Now, imagine you are in northern Florida during the past several Januarys. In the last few years, they stomached 10 consecutive nights with freezing temperatures. Or, how about in Texas, where last February they experienced two weeks of some of the worst cold and windy weather ever. The latter reduced daily gains from birth for 90-day-old calves at one ranch by 0.4 pound for those calves born during those two weeks. It is happening, but unless we are measuring body weight gains regularly, we may not “see” it in the calves.

So how does this all net out?

First, develop the best approach to adjust the liquid feeding program for your calves in colder weather. Remember, for each 10° F drop in average daily temperature below 60° F, calves will lose about 0.2 pound daily gain.

For the younger calves, adjustments need to occur in the liquid feeding program. For the bigger calves, that may require some additional energy through the liquid feeding program. But, it also requires greater attention to fostering starter intake which, in turn, requires more attention to feeding warm water.